We present [ C/N ] - [ Fe/H ] abundance trends from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment ( APOGEE ) survey , Data Release 14 ( DR14 ) , for red giant branch stars across the Milky Way Galaxy ( MW , 3 kpc < R < 15 kpc ) .
The carbon-to-nitrogen ratio ( often expressed as [ C/N ] ) can indicate the mass of a red giant star , from which an age can be inferred .
Using masses and ages derived by Martig et al. , we demonstrate that we are able to interpret the DR14 [ C/N ] - [ Fe/H ] abundance distributions as trends in age- [ Fe/H ] space .
Our results show that an anti-correlation between age and metallicity , which is predicted by simple chemical evolution models , is not present at any Galactic zone .
Stars far from the plane ( | Z | > 1 kpc ) exhibit a radial gradient in [ C/N ] ( \sim - 0.04 dex/kpc ) .
The [ C/N ] dispersion increases toward the plane ( \sigma _ { [ C / N ] } = 0.13 at | Z | > 1 kpc to \sigma _ { [ C / N ] } = 0.18 dex at | Z | < 0.5 kpc ) .
We measure a disk metallicity gradient for the youngest stars ( age < 2.5 Gyr ) of - 0.060 dex/kpc from 6 kpc to 12 kpc , which is in agreement with the gradient found using young CoRoGEE stars by Anders et al .
Older stars exhibit a flatter gradient ( - 0.016 dex/kpc ) , which is predicted by simulations in which stars migrate from their birth radii .
We also find that radial migration is a plausible explanation for the observed upturn of the [ C/N ] - [ Fe/H ] abundance trends in the outer Galaxy , where the metal-rich stars are relatively enhanced in [ C/N ] .